This proposal aims to obtain a more detailed understanding of basic principles underlying olfactory system function, organization, and development. Mechanisms of signal transduction, patterns of neuronal organization, and fundamental rules governing olfactory system development may be incisively addressed in simple olfactory systems consisting of a limited number of neurons, in organisms which can be investigated by a variety of forceful means. The Drosophila olfactory system is such a system, as it consists of approximately 103 neurons, and is amenable to convenient genetic, molecular, behavioral, and physiological analysis. This proposal calls for a detailed analysis of a gene, acjo, which is required for normal olfactory physiology. An acjo mutation severely affects response to a variety of odorants, but not to benzaldehyde (odor of almond). The odorant-specificity of the acjo mutation will be examined in detail, with a view to determining whether the mutation distinguishes among compounds based on simple structural criteria; these data may provide information regarding the logic of chemosensory coding. Further genetic and molecular analysis of acjo is proposed with an interest in determining whether acjo is expressed in a subset of the olfactory system. If acjo is in fact required for response only to a subset of odorants, then defining its expression pattern may reveal whether different odorant pathways are spatially segregated in the olfactory system. Molecular analysis is also proposed for two genes encoding putative odorant-binding proteins, both located at the same chromosomal site; if time and resources permit, a genetic analysis will be initiated. The tissue distribution of a new phospholipase C gene, recently amplified from an antennal RNA preparation, will be defined. Homologues of the putative vertebrate odorant receptors will also be sought.